Etchant for molybdenum



CROSS REFERENCE United States Patent 3,098,043 ETCHANT FOR MOLYBDENUM Douglas C. Wendell, Jr., Malvern, Pa., assignor to Burroughs Corporation, Detroit, Mich., a corporation of Michigan No Drawing. Filed Aug. 17, 1961, Ser. No. 132,018 6 Claims. (Cl. 252-795) This invention is a continuation-in-part of an application entitled Etchant for Molybdenum, Serial No. 848,233, filed on October 23, 1959, and now abandoned.

This invention relates to a composition or solution for the rapid etching of molybdenum.

A standard formulation for etching thin molybdenum sheet consists of an aqueous solution of 360 grams per liter of potassium ferricyanide and 36 grams per liter of sodium hydroxide; this solution is ordinarily applied by spraying or splashing upon the material to be etched, the solution temperature being maintained in the vicinity of 200 degrees Fahrenheit. Neither the concentration of chemicals nor the temperature is at all critical; therefore, a solution of 0.3 mole per liter of potassium ferricyanide and 0.25 mole per liter of sodium hydroxide and a temperature of 120 degrees Fahrenheit are also quite satisfactory. However, in both instances the rate of etching is quite slow, from twenty to thirty minutes being required to etch away 2 mils of molybdenum.

Among the objects of the present invention is the provision of a composition of matter or solution for effectively accelerating the etching of molybdenum.

I have found that the addition of sodium oxalate to the etchant reduces the etching time very markedly. For example, a solution of optimum composition is the folowing:

Moles per liter of solution Potassium ferricyanide .6 to .8 Sodium hydroxide .5 to .63 Sodium oxalate .022 to .045

Moles per liter of solution Potassium ferricyanide 1.5

Sodium hydroxide 1.25 Sodium oxalate .03

was found to etch almost as rapidly.

Another composition suitable for the practice of this invention is the following:

Moles per liter of solution Potassium ferricyanide .6 to .8 Potassium hydroxide .5 to .63 Potassium oxalate .022 to .045

When light-sensitive resists or other types of protective agents are employed in connection with an etching process, the attack of the etchant upon the protective agent or resist is a concomitant which frequently occurs to an undesired degree. In the practice of my invention, the addition of the etching accelerator does not noticeably increase the rate of attack of the etchant upon the protective agent or resist, and therefore the overall lowering of the etching time materially lessens the opportunity for the etchant to penetrate the protective agent or resist. The reduction in injury to the resist also permits the utilization of more violent mechanical agitation or splashing thereby resulting in more sharply defined etched boundaries in the finished product.

It is the oxalate ion which produces the beneficial effects I have observed. This may be deduced from the Patented July 16, 1963 fact that the alkali ion added by the addition of sodium oxalate is negligible by comparison with the amount of alkali ion already present. It is thus clear that it is quite immaterial whether the metallic ions present be of any particular chemical family provided their nature is consistent with the solubility of all the negative ions contained by the described etchant, and with the alkalin i-ty normally occurring in etchant of the described com position. As a practical matter, the cheaper and more readily available alkali metals will, of course, be preferred, since the alkaline earths (one of which is a famed source of cheap alkali) are noted for the insolubility of their oxalates. Oxalate ion may be provided by the addition of oxalic acid and an equivalent amount of the alkali chosen, instead of the oxalate as such.

An increase in the temperature of the solution will increase the etching speed, but may produce problems of evaporation of the solvent. The extremely high alkalinity of the solution tends to foaming in a mechanically agitated etcher such as a splash etcher. I have found that from one-tenth to three-tenths of a milliliter of a foam reducer, such as diethylene glycol monostearate added to each liter of etchant, will reduce foaming without impairment of the etching action.

It is part of the known art that the surfaces to be etched must be free from contaminations such as grease which would tend to shield them from the etchant.

Reduced concentrations of the etchant chemicals will reduce the speed of etching so much that the improvement produced by the addition of oxalate ion according to my invention will only be mildly effective; consequently, the use of extremely dilute solutions is not desirable. On the other hand, too high a concentration impairs the uniformity of etching, and tends somewhat to cause undercutting beneath a protective resist. If

EXAMlNER these effects are not objectionable, such high eoncentra tions may be used.

In the light of these desiderata, the following has proven to be a practical solution of reduced concentration:

Moles per liter Potassium ferricyanide .091 Sodium hydroxide .075 Sodium oxalate .00375 Stated in ionic terms this solution supra would be expressed as follows:

Moles per liter Fe(CN) (ferricyanide ion) .091 (OH) (hydroxide ion) .075

(C O (oxalate ion) .00375 Similarly, in certain applications, the following has proven to be a practical solution of high concentration:

Moles per liter Potassium ferricyanide 1.5 Sodium hydroxide 1.25 Sodium oxalate .045

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced other than as specifically described and illustrated.

and .00375 to .045 mole per liter of an alkali metal oxalate for use as an etching accelerator.

3. An aqueous bath for etching molybdenum comprising water, .091 mole per liter of potassium ferricyanide, .075 mole per liter of sodium hydroxide and .00375 mole per liter of sodium oxalate;

4. An aqueous bath for etching molybdenum comprising water, 1.5 moles per liter of potassium ferricyanide, 1.25 moles per liter of sodium hydroxide, and .045 mole per liter of sodium oxalate.

5. An aqueous solution for etching molybdenum comprising water, potassium ferricyanide in the range of concentration .6 mole to .8 mole per liter, sodium hydroxide in the range of concentration .5 mole to .63 mole per liter, and sodium oxalate in the range of concentration .022 to .045 mole per liter.

6. An aqueous solution for etching molybdenum comprising water, potassium ferricyanide in the range of concentration .6 to .8 mole per liter, potassium hydroxide in the range of concentration .5 to .63 mole per liter, and potassium oxalate in the range of concentration .022 to .045 mole per liter.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN AQUEOUS SOLUTION FOR ETCHING MOLYBDENUM COMPRISING WATER, 0.91 TO 1.5 MOLES PER LITER OF AN ALKALI METAL FERRICYANIDE, .075 TO 1.25 MOLES PER LITER OF AN ALKALI METAL HYDROXIDE AND .00375 TO .045 MOLES PER LITER OF AN ALKALI METAL OXALATE. 